A display device can have a touch sensor function by incorporating an optical sensor including a thin film diode into it. The data entry system of such a display device is based on the detection of the change in the light projected through the display device surface on the side of the user (i.e., display surface), which occurs when the display surface is touched with a finger or a touch pen.
With such a display device, the change in the light due to the touch on the display surface with a finger or the like can be small depending on the surrounding condition such as the ambient brightness. Therefore, in that case, the change in the light cannot be detected accurately by the optical sensor.
Japanese Patent Application Laid-Open Publication No. 2008-287061 discloses a technology that improves the light detection sensitivity of an optical sensor of a semiconductor device for use in a liquid crystal display device. This is described with reference to FIG. 8.
This semiconductor device has, over the substrate (active matrix substrate) 910, insulating layers 941, 942, 943, and 944, which are formed sequentially, a thin film diode 920, and a thin film transistor 930. The thin film diode 920 is a PIN-type diode having a semiconductor layer 921 composed of an n-type region 921n, a p-type region 921p, and a low-resistance region 921i. The thin film transistor 930 has a semiconductor layer 931 composed of a channel region 931c, an n-type region 931a as the source region, an n-type region 931b as the drain region. A gate electrode 932 is disposed facing the channel region 931c through an insulating layer 943. The n-type region 931b is connected to the pixel electrode (not shown).
The thin film diode 920 receives the light projected from the display side (upper side of FIG. 8). On the other hand, in order to block the light from the backlight (not shown) disposed on the other side of the substrate 910, which is the side opposite from the display (lower side of FIG. 8), from entering the thin film diode 920, a light-shielding layer 990 is provided between the thin film diode 920 and the substrate 910. The light-shielding layer 990 is formed to extend along the surface of a recess 992 formed by partially removing the insulating layer 941. Because the recess 992 tapers outward toward the top, the light-shielding layer 990 has a sloped surface 991 extending along the sloped surface of the recess 992.
The light-shielding layer 990 also functions as a reflective layer. Therefore, the light that entered through the display surface and then, instead of entering the thin film diode 920, fell between the thin film diode 920 and the light-shielding layer 990 is reflected by the light-shielding layer 990 and goes back to enter the thin film diode 920. The sloped surface 991 of the light-shielding layer 990 reflects the light that falls on the sloped surface 991 toward the thin film diode 920.
In the semiconductor device shown in FIG. 8, with the light-shielding layer 990 described above, more light that enters through the display surface is guided into the thin film diode 920. As a result, the light detection sensitivity can be improved.